Process for Surface Modifications of Tio2 Particles and Other Ceramic Materials

ABSTRACT

A method of preparing a surface modified ceramic material.

This application is a continuation of and claims priority to U.S.application Ser. No. 10/806,698 filed Mar. 23, 2004, the entire contentsof which is incorporated herein by reference.

The present invention relates to a process for making surface-modifiedparticles of titanium dioxide and ceramic materials, parts of theprocess and the products of the process. The surface is modified byproviding a layer of an inorganic compound. The layer of inorganiccompound may be further treated with a dopant selected from an organiccompound or an inorganic compound. The method is particularly effectivewhen the titanium dioxide particles or ceramic material has a size lessthan 100 nm. Such particles are referred to as nano-sized particles.

Coatings or surface modifications of particles are often useful toimpart special properties. An example of a particulate product that isoften subjected to surface modification is TiO₂ in the anatase form.Anatase TiO₂ is strongly photocatalytic. This photocatalytic effect maybe desirable, for example, for the removal of organic pollutants, but itwill be undesirable when it leads to the decomposition of organicsubstrates or supports. Surface modification makes it possible tomodulate the photocatalytic activity and other surface characteristics.

A number of different methods exist to apply surface modifications toceramic particles and give them special properties. For example, U.S.Pat. No. 6,440,383 discloses a method to produce ultrafine TiO₂particles. The method describes the use of additives to adjust theproperties of the resulting TiO₂ particles. These additives may appearas a surface modification on the TiO₂ particles. For example, whenphosphoric acid or a phosphate salt additive is used, particles with acoating of titanium pyrophosphate or another phosphate compound areobtained.

Processes have also been proposed for multiple-layer coatings on TiO₂.For instance, U.S. Pat. No. 6,291,067 discloses a method to coatparticles with a porous layer of calcium phosphate and an anionicsurface-active agent. The result is a product that still shows goodphotocatalytic activity towards decomposing organic matter whileprotecting the support on which the particles are placed. Other existingmethods involve reacting titanium dioxide pigment with aluminumphosphate (U.S. Pat. No. 5,785,748), or adding consecutive layers ofsilica (or zirconia) and alumina between phosphate layers (U.S. Pat. No.6,342,099).

It has now been found that new products with adjustable properties canbe obtained by starting with TiO₂ made according to the processdescribed in U.S. Pat. No. 6,440,383. Alternatively, a similar TiO₂ orother ceramic material can be used as the starting material. The termceramic material as used in the following specification and claimsrefers to a hard, brittle, heat and corrosion-resistant compound thatincludes one or more metals in combination with oxygen, the manufactureof which involves a firing or calcination step. This starting compoundmay be subjected to a succession of surface modification steps with adopant selected from the group consisting of an organic compound and aninorganic compound.

According to the process described in U.S. Pat. No. 6,440,383, ultrafineTiO₂ is made with phosphoric acid introduced as an additive. Theaddition of phosphate in the feed solution creates, in situ, a very thintitanium phosphate layer on the surface of the nanoanatase particleduring the calcination step.

The TiO₂ produced according to U.S. Pat. No. 6,440,383 may be used as astarting material in the process according to the present invention.This TiO₂ starting material is dispersed in an aqueous slurry after thewet milling step. The pH of the slurry is increased by addition of astrong base such as KOH. The strong base reacts with the titaniumphosphate surface layer and transforms it into a structure composed of awater insoluble titanate. In the case of KOH, the layer will have theapproximate composition K₂TiO₃. Water-soluble phosphate ions (potassiumphosphate) move into the solution. The particles at this stage of theprocess, with a layer of K₂TiO₃ on the surface, may be dried andcalcined.

The TiO₂ particles with the K₂TiO₃ layer may be acidified by addition ofHCl or another strong acid that is able to decompose potassium titanate.Water-soluble potassium salts, such as KCl in this case, migrate backinto the solution and the TiO₂ particle surface transforms into agelatinous hydrate of approximate formula Ti(OH)₄. The product at thisstage of the process may be dried and used as a titanium oxide with ahigh surface area. Alternatively, the dried product may be furthercalcined to make crystalline TiO₂ in the anatase form. The product ofthe calcination may be further covered by an activated layer, preferablya layer of a metal.

The method of the present invention is particularly effective when theparticles are nano-sized, because smaller particles have a relativelylarger surface area, i.e. the number of molecules placed at the surfaceis larger compared to the total number of molecules, and any surfaceeffect will be enhanced.

Following another aspect of the present invention, the product afteracidification and washing may be further treated with a dopant that willattach to the external gel on the TiO₂ particle. The dopant may beselected from the group consisting of an inorganic compound and anorganic compound. The inorganic compound may be selected from the groupconsisting of metals, colloidal metals, and metal salts. The metals maybe selected from Ag, Au, Cu, Zn, Pt, Sn, W, V, Y, and Mn. When the metalis Ag, Au, Cu, Zn, or Pt, the metal is preferably added in colloidalform or as a dissolved metal salt. If the dopant is a colloidal metal,the product is dried, then gently calcined to form a metal/anatase layeron an anatase particle. In the case where the dopant is a thermallyunstable salt of a metal as Sn, W, V, Y, or Mn, the product can becalcined to create a hard shell of oxide on the TiO₂ particle.

Suitable organic compounds include water-soluble organic compoundsincluding but not limited to carboxylic acids, carboxylic acid salts,and alcohols.

Although the above process contemplated the use of TiO₂ made accordingto the process described in U.S. Pat. No. 6,440,383, a TiO₂ base made byany process, and modified with titanium phosphate, can be used.Particles of other compounds such as aluminum oxide, zirconium oxide orsimilar ceramic compounds that are resistant to strong acids and strongbases can also be used as the base material for the titanium phosphatedeposition and the further steps described above.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a general schematic of one embodiment of the processaccording to the present invention.

FIG. 2 shows a schematic of a portion of the method of the presentinvention where a strong base reacts with a titanium pyrophosphate layeron anatase titanium dioxide to form a titanate.

FIG. 3 shows a schematic of a portion of the method of the presentinvention, where HCl reacts with the titanate formed in the step shownin FIG. 2.

FIG. 4 shows a schematic of a portion of the method of the presentinvention, where the coating on the particle corresponding to FIG. 3 isfurther subjected to a doping step.

FIG. 5 a shows a schematic of a portion of the method of the presentinvention, where the particle corresponding to FIG. 4 is furthersubjected to drying and calcination. FIG. 5 b shows a scanning electronmicrograph of a final product of nanoanatase particle surface modifiedby a zirconium oxide layer.

FIGS. 6 a-6 d show scanning electron micrographs for a base material andits surface modifications. FIG. 6 a is a 20 nm nanoanatase base. FIG. 6b is a 20 nm nanoanatase base with a SnO₂-modified surface. FIG. 6 c isa 40 nm nanoanatase base material. FIG. 6 d is a 40 nm nanoanatase baseafter surface modification with silver.

FIG. 7 shows the rate constant of different surface-treated materialsmade according to the process of the present invention when thematerials are used to decompose an organic material throughphotocatalytic action.

FIG. 8 shows cyclic voltamograms. FIG. 8 a shows the cyclic voltamogramof the 20 nm nanoanatase base before surface modification. FIG. 8 bshows the cyclic voltamogram of a 20 nm nanoanatase base after lithiumtitanate surface modification that shows a significant electrochemicalresponse.

FIG. 9 shows the cyclic voltamogram of 20 nm nanoanatase base aftersurface modifications, which do not exhibit a noticeable electrochemicalresponse. FIG. 9 a shows a 20 nm nanoanatase base with a SnO₂ surfacemodification. FIG. 9 b shows a 20 nm nanoanatase base with an anatasesurface modification. FIG. 9 c shows a 20 nm nanoanatase base with anAl₂O₃ surface modification. FIG. 9 d shows a 20 nm nanoanatase base witha Y₂O₃ surface modification. FIG. 9 e shows a 20 nm nanoanatase basewith a potassium titanate surface modification.

FIG. 10 shows the photochemical performance of surface modified samplesused in a Graetzel Cell setup. FIG. 10 a corresponds to an activatedanatase surface modification. FIG. 10 b corresponds to a 20 nmnanoanatase base with lithium titanate surface modification. FIG. 10 ccorresponds to a 20 nm nanoanatase base with potassium titanate surfacemodification. FIG. 10 d corresponds to a 20 nm nanoanatase base with tindioxide surface modification.

FIG. 11 a shows details of 4-CP photodegradation on 20 nm nanoanatasemodified by a layer of barium titanate.

FIG. 11 b shows details of 4-CP photodegradation on the same material asthat shown in FIG. 11 a, taken before treatment with the phosphatelayer.

DETAILED DESCRIPTION

Turning now to FIG. 1, a general schematic process flow sheet of oneembodiment according to the present invention is shown. Referring moreparticularly to FIG. 2, the initial portion of the process showngenerally in FIG. 1 is shown. For ease of description, the process willbe described with reference to the production of surface modified TiO₂particles. It is to be understood, however, that the process may includestarting with ceramic materials such as alumina or zirconia particlesthat have been coated with a layer of titanium phosphate or titaniumpyrophosphate.

TiO₂ particles with a layer of phosphate or a layer of pyrophosphate onthe surface may be made by any suitable process in one or more steps.One suitable process for making TiO₂ particles with a layer of phosphateor a layer of pyrophosphate is described in U.S. Pat. No. 6,440,383,where the chemical additive is phosphoric acid or a phosphate, therelevant portions of which are incorporated herein by reference. Thelayer of titanium pyrophosphate made by this method is no more than 2 nmthick. If it is attempted to make a thicker layer, the layer willseparate from the TiO₂ base. This material will be referred to as thebase material.

The base material is subjected to further surface modification byimmersion in a strong base solution such as KOH, LiOH, NaOH, CsOH, andRbOH, or any hydroxide that produces a water insoluble titanate and asoluble form of phosphate by reaction with TiO₂.

For illustrative purposes, if the base is KOH, the reaction is believedto be of the form.

TiP₂O₇+8KOH→K₂TiO₃+2K₃PO₄+4H₂O

The layer of phosphate on the TiO₂ base material is replaced by atitanate of a strong base on the surface of the TiO₂ particle. Suchcompounds formed by the action of a strong base on a ceramic oxide aregenerally known to be gelatinous. The same treatment with a strong basecan be applied to other core materials such as alumina or zirconia, iftitanium phosphate or pyrophosphate is first deposited on the surface.

The particles with the gelatinous titanate layer may be washed withwater or a polar solvent in which the gelatinous layer is soluble anddried at a temperature from about 80° C. to about 250° C. to produce aTiO₂ core with a surface modified by an alkali metal titanate. Theproduct of the drying step may be calcined at a temperature betweenabout 250° C. and about 1000° C. The product resulting from thecalcination is a dense titanate layer on the surface.

Turning now to FIG. 3, further steps of the general process depicted inFIG. 1 are shown. The product of the treatment by a base may be furthersubjected to treatment by a strong acid, such as hydrochloric acid. Thestrong acid attacks the layer of titanate of the strong base and leavesa gelatinous layer of hydrated titanium hydroxide. The reaction for theK⁺/HCl case may be written

K₂TiO₃+2HCl→2KCl+Ti(OH)₄

After the KCl has been rinsed out with water or a polar solvent, atitanium hydrate layer 1 to 20 nm thick is formed on the particle. Theproduct of this step may be dried at a temperature between about 80° C.and about 150° C. to produce a dry titanium hydrate. The dried productmay be further calcined at a temperature between about 300° C. and about800° C. Calcination will transform the hydrate into anhydrous TiO₂crystals. By removing hydration water, the calcination process shrinksand hardens the gelatinous layer of hydrated titanium.

Alternatively, after the KCl has been rinsed, the particle havingtitanium hydrate on its surface with water or a polar solvent can beused as an intermediate for the next step of the surface modificationprocess.

Referring to FIG. 4, a further possible step in the surface modificationprocess is shown. The intermediate is treated with a dopant, desirablywithout drying or calcining. The dopant is typically a water-solublecompound that can penetrate the gel layer on the base surface. Possibledopants can be divided into three categories: organic compounds,inorganic compounds, and colloids. For example, the dopant may be awater-soluble organic compound, an inorganic salt, particularly a metalsalt, or a colloidal metal or complex.

Possible organic compounds may be for example water-soluble amino acids,glycenes, and silenes. Possible inorganic compounds are metal salts orare any water or acid-soluble salt such as SnCl₄, BaNO₃, and AlCl₃.Possible colloids include metals such as Ag and Pt, or colloidalcomplexes. The dopants may be added as solutions or as colloidalsuspensions. By controlling the concentration of the dopant in thesolution, the final level of the dopant in the surface layer and thelayer thickness can be controlled.

The doped material may be used as a final product, or dried andprocessed further, as shown in FIG. 5 a. Referring to FIG. 5 a, thedopant fixed in the dried titanium hydrate is converted into anothercompound by drying and calcination under a controlled atmosphere.Compounds of limited thermal stability used for surface doping can bedecomposed by calcination and can be converted to other compounds.Oxides of the metal used in the surface modification process may beconveniently obtained. It is also possible to convert the surfacemodification layer to a metal layer by calcining under reducingconditions.

Drying causes shrinkage of the gel layer containing the dopants. Manyproducts can originate from this stage. For example, Ti hydrate may becoated with a colloidal metal, a metal salt, an organic compound or acomplex compound.

The product can be further calcined, generally to create a dry saltsurface modification on the anatase or an oxidized salt layer on theparticle surface. Some salts, such as metal nitrates, chlorides,acetates, and sulfates, decompose at temperatures lower than the anatasephase conversion point and can be used for this purpose. FIG. 5 b showsa scanning electron micrograph of a final product of nanoanataseparticles, surface-modified by a zirconium oxide layer.

Characterization of Surface Modified Samples

The following methods have been used to characterize thesurface-modified products:

4-CP Photodegradation

To determine photocatalytic activity of the TiO₂ surface modifiedsamples, the kinetics of disappearance of 0.1 mM 4-chlorophenol (4-CP)in 60 mL of aerated aqueous suspension of the photocatalyst (1 g/L) weremonitored as a function of time. The apparatus consists of two coaxialquartz tubes placed in the middle of a steel cylinder with an aluminumfoil covering its inner wall. The inner quartz tube (diameter 24 mm,length 300 mm) was filled with the investigated suspension (60 ml) andmagnetically stirred. Cooling distilled water was circulating betweenthe inner and the outer quartz tube to keep a constant temperature of20° C. The source of monochromatic UV light (λ=365 nm) was a mediumpressure mercury lamp closed in glass filter bulbs (RVU, 125 W, TeslaHole{hacek over (s)}ovice, Czech Republic). The equipment was calibratedby ferrioxalate actinometry: the average light intensity entering thevolume of 50-70 mL of irradiated solution was determined as I₀=4.9×10⁻⁵Einstein L⁻¹ s⁻¹. Probes of irradiated suspensions (0.5 mL) were takenat appropriate times. The solid photocatalyst (together with theadsorbed portions of the dissolved molecular species) was removed beforeanalysis by liquid chromatography (HPLC), employing filtration through aMillipore syringe adapter (diameter 13 mm) with filter 408 (porosity0.45 μm).

Ultra-Violet/Visible/Near Infrared Diffusion Reflectance(UV/Visible/Near IR).

UV/visible/near IR diffusion reflectance spectra were measured from 2500to 250 nm on a Perkin-Elmer Spectrophotometer Lambda 19 equipped with anintegrating sphere. Both powders and their 35% aqueous suspensions wereplaced into a 1 cm spectroscopic cell and scanned against BaSO₄ plate asa standard. To enable quantitative evaluations, the measured values ofreflectance versus wavelength were transformed into Kubelka-Munk unitvs. wave number coordinates. The Schuster-Kubelka-Munk theory gives theratio of the absorption (K) to the scattering (S) coefficients asK/S=(1−R∞)2/2 R∞) where R∞ is reflectance measured as the ratio of thetotal intensities of light reflected from the sample and from thestandard (BaSO₄). The liquid chromatography experiments were run on aMerck device with L-6200 Intelligent Pump, L-3000 Photo Diode ArrayDetector, and D-2500 Chromato-Integrator. Mobile phase methanol/water(2:3; v/v) and a Merck column LiChroCART 125-4 filled with LiChrosphere100 RP-18 (5 μm) were used, injection loop was 20 μL, flow rate 1 mLmin−1 and detection wavelength 280 nm were applied.

Electrochemical Tests

Pure TiO₂ film electrodes, prepared from corresponding surface modifiedmaterials were tested by cyclic voltametry in 1 M LiN(CF₃SO₂)₂electrolyte solution. Dark electrochemistry in Li⁺ containing solutionis a useful tool to characterize the structure and morphology ofelectrodes from nanocrystalline anatase and can determine trace amountsof electrochemically responding materials. Electrochemical measurementswere carried out in a one-compartment cell using an Autolab Pgstat-20(Ecochemie) controlled by GPES-4 software. The reference and auxiliaryelectrodes were from Li metal, hence potentials are referred to theLi/Li⁺ (1M) reference electrode. In all cyclic voltametry experiments,the direction of the first potential sweep was (1) equilibrium potential(2) lower vertex potential (3) upper vertex potential. LiN(CF₃SO₂)₂(Fluorad HQ 115 from 3M) was dried at 130° C./1 mPa. Ethylene carbonate(EC) and 1,2-dimethoxyethane (DME) were dried over a 4A molecular sieve(Union Carbide). The electrolyte solution, 1 M LiN(CF₃SO₂)₂+EC/DME (1/1by mass), contained 10-15 ppm H₂O as determined by Karl Fischertitration (Metrohm 684 coulometer). All operations were carried out in aglove box.

Solar Cells and Photoelectrochemical Measurements

The sensitized electrodes prepared from the surface treated anatase wereassembled into solar cells. The test cells were thin-layer sandwich-typesolar cells with reflecting Pt counter electrode. The latter was a 2μm-thick Pt mirror, which had been deposited on SnO₂(F) conducting glassby vacuum sputtering. The counter-electrode was placed directly on topof the dye-coated TiO₂ film, supported by conducting glass sheet. Bothelectrodes were tightly clamped together. A thin layer of theelectrolyte was attracted into the inter-electrode space by capillaryforces. All tests employed the same electrolyte, gamma-3. Theelectrolyte gamma-3 is based on I⁻/I₃ ⁻ redox couple in γ-butyrolactonemedium.

The dye-coated TiO₂ film was illuminated through the conducting glasssupport. The light source was a 450 W xenon lamp that was focused togive light power of 1000 W/m² at the surface of test cell. The spectraloutput of the lamp was matched in the wavelength region of 350-750 nmwith the aid of a Schott KG-5 sunlight filter. This reduced the mismatchbetween the simulated and the true solar spectrum to less than 2%. Thediffering intensities were regulated with neutral wire mesh attenuator.The basic cell characteristics were measured at three different lightintensities, equivalent to 10% Sun, 50% Sun and 100% Sun. The appliedpotential and cell current were measured using a Keithley model 2400digital source meter. The current-potential characteristics of the cellunder these conditions were determined by biasing the cell externallyand measuring the generated photocurrent. The solar energy conversionefficiency, η, is defined as follows:

η=J _(pmax) ·U _(pmax) /P _(L)  (1)

J_(pmax) is the photocurrent density (in A/m² (projected electrodearea)), U_(pmax) is the cell voltage and P_(L) is the light intensity(in W/m²). P_(L) is the incident light power, determined in front of thewindow, which is not corrected for absorption and reflection losses inthe window, electrolyte solution, and electrode. The actual values ofJ_(pmax) and U_(pmax) are determined from the current-potentialcharacteristics of the cell, while their product J_(pmax)·U_(pmax) (thecell output power) is maximum. The ratio of such defined maximum outputpower to the product of maximum current density (short circuit currentdensity, J_(SC)) and the maximum cell's voltage (open circuit voltage,U_(OC)) gives the cell's fill factor, f:

f=J _(pmax) ·U _(pmax) /J _(SC) ·U _(OC)  (2)

A similar data-acquisition routine was used to measure the incidentphoton-to-current conversion efficiency (IPCE) with monochromatic light.Light from 300 W xenon lamp was focused through a high throughputmonochromator onto the solar cell under test.

As a rule, every material passed several photoelectrochemical tests toavoid casual experimental errors and to check the reproducibility ofmeasurement. Each individual fabrication run provided 10 electrodeswhose properties (layer thickness, mass, porosity etc.) were identical.The standard quadratic error in a set of parallel measurements on theseelectrodes from one batch was not larger than 7%.

After completing the photoelectrochemical tests, the solar cell wasdisassembled, and the photo electrode was carefully washed byacetonitrile and dried in air. Such electrode was subjected to twoadditional tests: (1) check of the layer thickness by alpha-stepprofilometer (2) measurement of the amount of adsorbed dye N-719. Theadsorbed N-719 was determined as follows:

The sensitized electrode was dipped into 3.00 mL of phosphate buffer (pH7) and the solution was stirred for about 5 minutes to complete thedesorption of N-719 into the buffer. The resulting solution of N-719 wasanalyzed spectrophotometrically in 1.00 cm quartz optical cell (Hellma).The concentration of N-719 was calculated using the following extinctioncoefficients (in M⁻¹ cm⁻¹):

53 000 (λ=308 nm); 12 700 (λ=372 nm); 13 600 (λ=500 nm)

The extinction coefficients were determined by using a fresh standard10⁻⁵ M solution of N-719. The solution was prepared by dissolving of1.55 mg of pure solid N-719 in 125 mL of phosphate buffer. (It isrecommended that the test and reference solutions of N-719 be preparedfreshly before each series of spectrophotometrical measurement. Thereason is that the optical densities are not stable during long-termstoring. Decrease by 28% was found after 27 days of storage of the 10⁻⁵M solution of N-719 at ambient conditions).

The surface coverage of a dye, Γ_(d) was determined by using the BETsurface areas provided for each material. Alternatively, the surfacecoverage was expressed as the relative area per one molecule of N-719(A_(M)). Both the Γ_(d), and A_(M) values assume two simplifications:(1) The surface area, “seen” by the N₂ molecule (in BET measurement) isidentical to the surface area “seen” by the N-719 molecule, although thelatter is considerably larger, hence, the N-719 molecule cannotpenetrate into very narrow pores (2) The BET area of a powder precursoris equal to the physical area of the electrode; in other words, the lossof surface area, caused by sintering of nanopowder, is neglected. Thelatter correction should be considered especially in small nanocrystals:it has been found that there is about a 15-37% loss of surface area ifnanocrystalline TiO₂ powder having 47-404 m²/g is sintered intoelectrode layers. Obviously, the above approximations give the lowerestimate for Γ_(d) and the upper estimate for A_(M).

Characterization Results

FIG. 7 shows the contrast in photocatalytic behavior between differentsurface-treated materials. The anatase core is the same in all samples,but the surfaces were treated with different compounds. In this figure,Sample S1 is TiO₂ anatase coated with a thin layer of titanium phosphateor pyrophosphate. This material served as basis for the surfacemodifications corresponding to the other samples. Sample S2 is titaniumhydrate without surface modification. Samples S3, S4, and S5 are TiO₂samples with surface modification by SnO₂, BaSO₄ and BaTiO₃respectively. S6 is a pure nano-anatase standard given as reference.

It is difficult to determine the chemical formula of a material,existing as few nanometer thin film on the base surface. XRD was used todetermine possible separation of phases, but it is not a goodcharacterization method to identify a chemical composition of an ultrathin layer on a particle surface. Chemical composition of some of thesematerials was determined by electrochemical characterization by cyclicvoltametry.

FIGS. 8 a and 8 b show voltamograms of the nanoanatase base materialbefore and after the nanoanatase base was surface-modified with lithiumtitanate. There is a clear electrochemical response on the graph,corresponding to the compound Li₄Ti₅O₁₂.

FIGS. 9 a-9 e show surface modifications with compounds that do notexhibit a significant electrochemical response.

FIGS. 10 a-10 d show incident photon to current conversion efficiencymeasurements of four surface-treated materials. Performance of each ofthese materials in a Graetzel Cell set up is vastly different whichunderscores the impact of the modified surface on the overall propertiesof the product.

The following Examples are meant to illustrate but not limit the presentinvention or claims.

EXAMPLE 1 Examples of Base Material Manufacturing

Method 1

A solution containing 120 g/l Ti and 400 g/l Cl was made by injectingtitanium chloride into a hydrochloric acid solution. Phosphoric acid wasadded to this solution in an amount corresponding to aphosphorous/titanium weight ratio of 0.04. The solution was fed to aspray dryer at a rate of 2.25 liters/min through an atomizing disk.Gases from the combustion of natural gas, diluted with air to 550° C.,were also injected around the disk. The outlet temperature was 250° C.and the total gas flow rate was about 800 scfm. Reactor off gases weresent to a bag filter to collect the TiO₂ product. The collected productwas further calcined at 800° C. for 8 hours then dispersed by wetmilling. The slurry produced by the milling operation was used as theanatase base material for surface modifications. FIG. 6 a shows ascanning electron micrograph of the product.

Method 2

A powder of Al₂O₃ was mixed with a solution of titanium phosphate andHCl and spray-dried. The flow and temperature conditions of the spraydrier were the same as those given in Method 1. The product of thespray-drying step was calcined at 900° C. and micronized. Thephotocatalytic activity was determined by the method described above andwas found to be low.

Method 3

A powder of ZrO₂ was mixed with a solution of titanium phosphate and HCland spray-dried under the same conditions as those described inMethod 1. The product of the spray-drying step was calcined at 900° C.and dispersed by wet milling. The photocatalytic activity of this basematerial was close to zero.

EXAMPLE 2

The nano anatase slurry product prepared by Method 1 of Example 1 wasmixed with a concentrated solution of lithium hydroxide, then thoroughlywashed with water and dried at 150° C.

The composition of the new surface obtained by this procedure wasdetermined by cyclic voltametry to be Li₄Ti₅O₁₂, as shown in FIGS. 8 aand 8 b. This Li₄Ti₅O₁₂ surface-modified sample was photochemicallycharacterized in the Graetzel Cell set up described above. Thesemiconducting properties of the anatase core were shielded by theLi₄Ti₅O₁₂ coating. The energy conversion was very poor, as shown in FIG.10 b, most likely because of electron transfer through the nonconductivelithium titanate layer on the anatase surface.

EXAMPLE 3

The nanoanatase slurry product prepared by Method 1 of Example 1 wasmixed with a concentrated solution of potassium hydroxide, thenthoroughly washed, dried at 150° C. and calcined at 500° C.

The exact chemical composition of the new surface obtained by thisprocedure is not known. It is assumed that a hard shell of potassiumtitanate of approximate formula K₂TiO₃ formed. The potassium surfacemodified sample was photochemically characterized in the Graetzel Cellsetup. FIG. 10 c shows that the semiconducting properties of nanoanatasewere completely blocked by the K₂TiO₃ layer.

EXAMPLE 4

The nanoanatase product prepared by Method 1 of Example 1 was leached inKOH, washed in de-ionized water, then immersed in HCl and washed againin de-ionized water, until the pH reached 4-5. The result is agelatinous coating that was dried at 150° C. The rate of decompositionof 4-CP decreased by a factor of about 2 compared to the rate observedwith untreated anatase TiO₂ (see FIG. 7).

EXAMPLE 5

The product of Example 4 was further calcined at 500° C. for 2 h. Therate of decomposition of 4-CP increased compared to Example 4.Nanoanatase with this surface treatment also significantly improved theperformance of the Graetzel Cell (see FIG. 10 a). The electrochemicalbehavior of this product is shown in FIG. 9 b (labeled as sample SL28).

EXAMPLE 6

Nano-sized anatase prepared by Method 1 of Example 1 was leached in KOH,washed in de-ionized water, then immersed in HCl solution and washedagain in de-ionized water, until the pH reached 1-2. The product of thistreatment has a gel-like surface made of a titanium hydrate. Thisproduct was further doped with tin chloride, spray dried, and calcinedat 800° C. to form a shell of SnO₂ on the surface of the core made ofnanosized anatase, (see FIG. 6 b).

The rate of decomposition of 4-CP increased compared to the previousexample (compare sample S3 with sample S2 in FIG. 7) and this surfacetreatment shows an improved performance in the Graetzel Cell applicationas shown in FIG. 10 d). The SnO₂, presumably present as a hard shellsurface modification did not produce any additional peaks on the cyclicvoltamogram. Accordingly, it appears that this coating is substantiallytransparent to Li⁺ ions migrating in and out of the anatase core.

EXAMPLE 7

The nano-sized anatase slurry product prepared by Method 1 of Example 1was mixed with a concentrated solution of hot barium hydroxide,thoroughly washed, dried at 150° C., and calcined at 800° C. The exactchemical composition of the new surface obtained by this procedure isnot known. It is assumed that a hard shell of barium titanate ofapproximate formula BaTiO₃ is formed. The rate constant for the reactionof 4-CP in the presence of this sample is given in FIG. 7, sample S5.The 4-CP disappearance was slower than for the reference sample ofuntreated anatase TiO₂. One can also see, by comparing FIGS. 11 a and 11b, that the amount of intermediate products produced duringphotodecomposition of the 4-CP is much smaller in the case of a bariumtitanate layer than in the case of the nanoanatase base material.

EXAMPLE 8

The product prepared by Method 1 was leached in KOH, washed in DI water,then immersed in HCl and washed again in DI water until the pH reached 4to 5. The result is a gelatinous coating of titanium hydrate asdescribed in Examples 4 and 6. This coating was further saturated withcolloidal platinum and gently dried.

EXAMPLE 9

The washed nanoanatase with the gelatinous surface, prepared asdescribed in Example 4 was further saturated with ascorbic acid indarkness and gently dried. After the dried product was exposed todaylight, it turned brown, as the organic compound was photo-oxidized onthe anatase surface.

EXAMPLE 10

The washed nanoanatase with the gelatinous surface, prepared asdescribed in Example 4 was further slurried in an aqueous solution ofKH₂PO₄ and spray-dried. The resulting spray drier discharge was calcinedat 500° C. for 5 h. A scanning electro micrograph of the product showedthat about 10% of the KH₂PO₄ was present as a thin layer on the surfaceof the anatase. The thickness of the layer was estimated at less than0.5 nm.

While there have been described what are presently believed to be thepreferred embodiments of the invention, those skilled in the art willrealize that changes and modifications may be made without departingfrom the spirit of the invention. It is intended, therefore, to claimall such changes and modifications that fall within the true scope ofthe invention.

1. A process for making a surface-modified material comprising: a.mixing a base material and a phosphate material wherein one of the basematerial and the phosphate material includes titanium to form a mixture;b. spray drying the mixture to form a base material with surface havinga layer of a titanium phosphate; c. treating the product of step b witha strong base; and d. washing the product of step c.
 2. The process ofclaim 1 further comprising drying the product after washing.
 3. Theprocess of claim 2 further comprising calcining the dried product. 4.The process of claim 1 further comprising: a. contacting the washedproduct with a strong acid; and b. subsequently washing the product toform a gelatinous layer of titanium oxide hydrate on the base material.5. The process of claim 4 further comprising drying the product with thegelatinous layer.
 6. The process of claim 5 further comprising calciningthe dried product.
 7. The process of claim 4 further comprising treatingthe product with a dopant.
 8. The process of claim 5 further comprisingsequentially treating the dried product with a dopant and then drying.9. The process of claim 8 further comprising calcining the driedproduct.
 10. The process of claim 1 where the base material is titaniumdioxide.
 11. The process of claim 1 where the base material is selectedfrom the group consisting of a ceramic metal oxide, a ceramic mixedoxide, a mixed metal oxide, or a mixture thereof.
 12. The process ofclaim 11, where the ceramic metal oxide is selected from the groupconsisting of TiO₂, ZrO₂, Al₂O₃, and mixtures thereof.
 13. The processof claim 8 where the dopant is a noble metal resistant to strong baseand strong acid.
 14. The process of claim 12 where the ceramic metaloxide consists of nano-sized particles with a size between 20 and 100nm.
 15. The process of claim 1 where the strong base is KOH.
 16. Theprocess of claim 4 where the strong acid is HCl.
 17. The process ofclaim 7 wherein the dopant is selected from the group consisting of acolloidal metal, a colloidal complex, an organic compound, an inorganicsalt, and mixtures thereof.
 18. The process of claim 4 furthercomprising: a. saturating the surface with a water-soluble organic orinorganic compound; b. subsequently drying; and c. calcining.
 19. Theprocess of claim 4 further comprising: a. saturating the surface with acolloid material; b. subsequently drying; and c. calcining.
 20. Theprocess of claim 17 wherein the dopant is a colloidal metal and thefinal product has a thin metal oxide surface layer.